Boost reservoir egr control

ABSTRACT

Methods and systems are provided for reducing turbo lag in a boosted engine. A boost reservoir coupled to the engine may be charged with compressed intake air and/or combusted exhaust gas. The pressurized charge may then be discharged during a tip-in to either the intake or the exhaust manifold.

TECHNICAL FIELD

This application relates to the field of motor-vehicle engineering, andmore particularly, to air induction in motor vehicle engine systems.

BACKGROUND AND SUMMARY

A boosted engine may offer greater fuel efficiency and lower emissionsthan a naturally aspirated engine of similar power. During transientconditions, however, the power, fuel efficiency, and emissions-controlperformance of a boosted engine may suffer. Such transient conditionsmay include rapidly increasing or decreasing engine load, engine speed,or mass air flow. For example, when the engine load increases rapidly, aturbocharger compressor may require increased torque to deliver anincreased air flow. Such torque may not be available, however, if theturbine that drives the compressor is not fully spun up. As a result, anundesirable power lag may occur before the intake air flow builds to therequired level.

It has been recognized previously that a turbocharged engine system maybe adapted to store compressed air and to use the stored, compressed airto supplement the air charge from the turbocharger compressor. Forexample, Pursifull et al. describe a system in US 2011/0132335 whereincompressed air is stored in a boost reservoir and is dispensed into theintake manifold when insufficient compressed air is available from theturbocharger compressor. In particular, the boost reservoir is chargedwith fresh intake air and/or effluent from one or more unfueledcylinders. By dispensing extra compressed air from the boost reservoirto the intake manifold, torque corresponding to the dispensed air can beprovided to meet the torque demand while the turbine spins up.

However, the inventors herein have identified potential issues with sucha system. As an example, during selected boost conditions, such as whenan operator tips-in while the boost level is above a threshold, EGR maybe requested but not available as rapidly as required. Specifically,recirculated exhaust gas may not be immediately available via lowpressure EGR due to the slower response time of the low pressure EGR.Recirculated exhaust gas may also not be immediately available via highpressure EGR due to the pressure difference between the intake andexhaust manifolds which would cause the high pressure EGR to flowbackwards into the exhaust manifold. Consequently, increased EGR may notbe possible and engine performance during those selected boostedconditions may be degraded.

Some of the above issues may be at least partially addressed by a methodfor a turbocharged engine. In one embodiment, the method comprises,during lower boost conditions, charging a boost reservoir with at leastcombusted exhaust gas to a first pressure; during higher boostconditions, raising the boost reservoir to a second, higher pressure byfurther charging the boost reservoir with compressed intake gas; andthen discharging the pressurized gas from the boost reservoir to anintake manifold. In this way, exhaust gas may be pre-stored in a boostreservoir for subsequent recirculation, and further can be increased toa sufficient pressure (even above the highest exhaust pressure) so thatsufficient exhaust gas can be delivered to the intake when desired (evenwhen the intake is highly boosted).

For example, during boosted engine operation, while the boost level isbelow a threshold, a boost reservoir may be charged with at least someexhaust gas at lower pressure from the exhaust manifold, thus providinga source of stored EGR. In response to a tip-in (e.g., towards wide openthrottle), while the boost level is above the threshold, high pressureEGR may be immediately requested. To provide the higher pressure EGR,compressed intake gasses can be added to the boost tank to raise thepressure of exhaust gasses already stored therein. In this way, eventhough the exhaust pressure may not be high enough to charge the tankwith exhaust gas at sufficient pressure for delivery to the intake, theaddition of higher pressure intake gasses can raise the pressure, thusallowing at least some exhaust gas to be delivered to the intake, evenwhen highly boosted. The discharged high pressure EGR may enableimproved combustion control and reduced NOx emissions. In this way,combusted exhaust gas may be pre-stored in a boost reservoir to latersupplement high pressure or low pressure EGR.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows aspects of an example engine system inaccordance with an embodiment of this disclosure.

FIG. 2 illustrates an example method for charging a boost reservoir withone or more of combusted exhaust gas and fresh intake air.

FIG. 3 illustrates an example method for discharging pressurized chargefrom a boost reservoir into an intake or an exhaust manifold.

FIG. 4 illustrates an example method for discharging pressurized chargefrom a boost reservoir to provide high pressure EGR.

FIG. 5 illustrates an example method for discharging pressurized chargefrom a boost reservoir to an intake manifold while pre-chargingcompressor boost pressure.

FIGS. 6-8 show example charging and discharging operations of a boostreservoir, according to the present disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for reducingturbo lag in a boosted engine including a boost air reservoir, such asin the engine system of FIG. 1. By discharging pressurized charge fromthe boost reservoir to the intake manifold or exhaust manifold inresponse to a tip-in, exhaust gas temperatures and pressures can bequickly raised, and a boosting device turbine can be rapidly spun-up. Anengine controller may be configured to perform a control routine, suchas the example method of FIG. 2, to charge the boost air reservoir withone or more of combusted exhaust gas from the exhaust manifold or freshintake air from the intake manifold, when charging opportunities areavailable. The controller may be further configured to perform a controlroutine, such as the example method of FIG. 3, to discharge thepressurized charge from the reservoir into the intake manifold and/orthe exhaust manifold based on engine operating conditions as well as thecomposition of charge available in the reservoir. When discharging tothe intake manifold, the controller may be configured to perform acontrol routine, such as the example method of FIG. 4, to dischargepressurized charge into the intake manifold from the reservoir whileholding an intake throttle closed, and then opening the throttle oncethrottle inlet pressures have been sufficiently raised. Thiscoordination allows throttle inlet pressures to be advantageously raisedwhile torque demand is met by charge discharged from the boostreservoir. As shown in FIG. 5, during selected boost conditions, whenhigh pressure EGR is requested, the controller may also be configured toraise a pressure of combusted exhaust gas stored in the reservoir bymixing it with compressed intake air, and then delivering the highpressure charge mixture to the intake manifold. Example charging anddischarging operations are shown with reference to FIGS. 6-8. Byincreasing exhaust temperature and pressures, turbine spin-up may beexpedited to reduce turbo lag. By using the boost reservoir to enablehigh pressure EGR to be provided during boosted operating conditions,boosted engine performance may be improved.

FIG. 1 schematically shows aspects of an example engine system 100including an engine 10. In the depicted embodiment, engine 10 is aboosted engine coupled to a turbocharger 13 including a compressor 14driven by a turbine 16. Specifically, fresh air is introduced alongintake passage 42 into engine 10 via air cleaner 12 and flows tocompressor 14. The compressor may be any suitable intake-air compressor,such as a motor-driven or driveshaft driven supercharger compressor. Inengine system 10, however, the compressor is a turbocharger compressormechanically coupled to turbine 16 via a shaft 19, the turbine 16 drivenby expanding engine exhaust. In one embodiment, the compressor andturbine may be coupled within a twin scroll turbocharger. In anotherembodiment, the turbocharger may be a variable geometry turbocharger(VGT), where turbine geometry is actively varied as a function of enginespeed.

As shown in FIG. 1, compressor 14 is coupled, through charge-air cooler18 to throttle valve 20. Throttle valve 20 is coupled to engine intakemanifold 22. From the compressor, the compressed air charge flowsthrough the charge-air cooler and the throttle valve to the intakemanifold. The charge-air cooler may be an air-to-air or air-to-waterheat exchanger, for example. In the embodiment shown in FIG. 1, thepressure of the air charge within the intake manifold is sensed bymanifold air pressure (MAP) sensor 24. A compressor by-pass valve (notshown) may be coupled in series between the inlet and the outlet ofcompressor 14. The compressor by-pass valve may be a normally closedvalve configured to open under selected operating conditions to relieveexcess boost pressure. For example, the compressor by-pass valve may beopened during conditions of decreasing engine speed to avert compressorsurge.

Intake manifold 22 is coupled to a series of combustion chambers 30through a series of intake valves (not shown). The combustion chambersare further coupled to exhaust manifold 36 via a series of exhaustvalves (not shown). In the depicted embodiment, a single exhaustmanifold 36 is shown. However, in other embodiments, the exhaustmanifold may include a plurality of exhaust manifold sections.Configurations having a plurality of exhaust manifold section may enableeffluent from different combustion chambers to be directed to differentlocations in the engine system.

In one embodiment, each of the exhaust and intake valves may beelectronically actuated or controlled. In another embodiment, each ofthe exhaust and intake valves may be cam actuated or controlled. Whetherelectronically actuated or cam actuated, the timing of exhaust andintake valve opening and closure may be adjusted as needed for desiredcombustion and emissions-control performance.

FIG. 1 shows electronic control system 38, which may be any electroniccontrol system of the vehicle in which engine system 10 is installed. Inembodiments where at least one intake or exhaust valve is configured toopen and close according to an adjustable timing, the adjustable timingmay be controlled via the electronic control system to regulate anamount of exhaust present in a combustion chamber during ignition. Theelectronic control system may also be configured to command the opening,closure and/or adjustment of various other electronically actuatedvalves in the engine system—throttle valves, compressor by-pass valves,waste gates, EGR valves and shut-off valves, various reservoir intakeand exhaust valves, for example—as needed to enact any of the controlfunctions described herein. Further, to assess operating conditions inconnection with the control functions of the engine system, theelectronic control system may be operatively coupled to a plurality ofsensors arranged throughout the engine system—flow sensors, temperaturesensors, pedal-position sensors, pressure sensors, etc.

Combustion chambers 30 may be supplied one or more fuels, such asgasoline, alcohol fuel blends, diesel, biodiesel, compressed naturalgas, etc. Fuel may be supplied to the combustion chambers via directinjection, port injection, throttle valve-body injection, or anycombination thereof. In the combustion chambers, combustion may beinitiated via spark ignition and/or compression ignition.

As shown in FIG. 1, exhaust from the one or more exhaust manifoldsections is directed to turbine 16 to drive the turbine. When reducedturbine torque is desired, some exhaust may be directed instead througha waste gate (not shown), by-passing the turbine. The combined flow fromthe turbine and the waste gate then flows through emission controldevice 70. In general, one or more emission control devices 70 mayinclude one or more exhaust after-treatment catalysts configured tocatalytically treat the exhaust flow, and thereby reduce an amount ofone or more substances in the exhaust flow. For example, one exhaustafter-treatment catalyst may be configured to trap NO from the exhaustflow when the exhaust flow is lean, and to reduce the trapped NO whenthe exhaust flow is rich. In other examples, an exhaust after-treatmentcatalyst may be configured to disproportionate NO or to selectivelyreduce NO with the aid of a reducing agent. In still other examples, anexhaust after-treatment catalyst may be configured to oxidize residualhydrocarbons and/or carbon monoxide in the exhaust flow. Differentexhaust after-treatment catalysts having any such functionality may bearranged in wash coats or elsewhere in the exhaust after-treatmentstages, either separately or together. In some embodiments, the exhaustafter-treatment stages may include a regenerable soot filter configuredto trap and oxidize soot particles in the exhaust flow.

All or part of the treated exhaust from emission control device 70 maybe released into the atmosphere via exhaust conduit 35. Depending onoperating conditions, however, some exhaust may be diverted instead toEGR passage 51, through EGR cooler 50 and EGR valve 52, to the inlet ofcompressor 14. In this manner, the compressor is configured to admitexhaust tapped from downstream of turbine 16. The EGR valve may beopened to admit a controlled amount of cooled exhaust gas to thecompressor inlet for desirable combustion and emissions-controlperformance. In this way, engine system 10 is adapted to provideexternal, low-pressure (LP) EGR. The rotation of the compressor, inaddition to the relatively long LP EGR flow path in engine system 10,provides excellent homogenization of the exhaust gas into the intake aircharge. Further, the disposition of EGR take-off and mixing pointsprovides very effective cooling of the exhaust gas for increasedavailable EGR mass and improved performance.

In engine system 10, compressor 14 is the primary source of compressedintake air, but under some conditions, the amount of intake airavailable from the compressor may be inadequate. Such conditions includeperiods of rapidly increasing engine load, such as immediately afterstart-up, upon tip-in, or upon exiting deceleration fuel shut-off(DFSO). As such, during a DFSO operation, fuel injection to one or moreengine cylinders is selectively deactivated responsive to selectedvehicle deceleration or braking conditions. During at least some ofthese conditions of rapidly increasing engine load, the amount ofcompressed intake air available from the compressor may be limited dueto the turbine not being spun up to a sufficiently high rotational speed(for example, due to low exhaust temperature or pressure). As such, thetime required for the turbine to spin up and drive the compressor toprovide the required amount of compressed intake air is referred to asturbo lag. During turbo-lag, the amount of torque provided may not matchthe torque demand, leading to a drop in engine performance.

In view of the issues noted above, engine system 100 includes boostreservoir 54. The boost reservoir may be any reservoir of suitable sizeconfigured to store pressurized charge for later discharge. As usedherein, the pressurized charge refers to the gas stored in reservoir 54.As such, the pressurized charge stored in boost reservoir may includeonly clean intake air (e.g., compressed intake air drawn from the intakemanifold), only combusted exhaust gas (e.g., combusted exhaust gasesdrawn from the exhaust manifold), or a combination thereof (e.g., amixture of intake air and exhaust gas having a defined EGR percentage).In one embodiment, the boost reservoir may be configured to store chargeat the maximum pressure generated by compressor 14. Various inlets,outlets, and sensors may be coupled to the boost reservoir, aselaborated below. In the embodiment shown in FIG. 1, pressure sensor 56is coupled to the boost reservoir and configured to respond to thecharge pressure there-within.

In engine system 100, boost reservoir 54 is selectably coupled to intakemanifold 22 upstream and downstream of intake throttle valve 20. Morespecifically, the boost reservoir 54 is configured to dischargepressurized charge to the intake manifold, downstream of the intakethrottle valve 20, via boost reservoir intake discharge valve 84. Theboost reservoir intake discharge valve may be a normally closed valvecommanded to open when a flow of charge from the boost reservoir to theintake manifold is desired. In some scenarios, the pressurized chargemay be delivered when the throttle valve is at least partially open.Therefore, check valve 94 may be coupled upstream of the throttle valveand oriented to prevent the release of pressurized charge from the boostreservoir backwards through the throttle valve. In other embodiments,the check valve may be omitted and other measures taken to preventbackwards flow through the throttle valve. In some embodiments, apressure recovery cone (not shown) may be fluidically coupled betweenthe boost reservoir and the intake manifold so that pressurized chargeis conducted through the pressure recovery cone on discharge from theboost reservoir. When included, the pressure recovery cone converts flowenergy back to pressure energy during flow conditions by suppressingflow detachment from the conduit walls. In alternate embodiments,however, the pressure recovery cone may not be included.

In still further embodiments, such as when the pressurized charge isbeing delivered to the intake manifold during boosted engine operatingconditions, the pressurized charge may be delivered with the intakethrottle valve held closed for a duration. As elaborated at FIG. 5, thethrottle may be held closed until the boost reservoir is fullydischarged or until a threshold throttle inlet pressure is attained.Then, the intake discharge valve can be closed while the intake throttlevalve is opened to allow compressed intake air from the compressor to bedischarged into the intake manifold. By temporarily holding the throttleclosed while the pressurized charge is discharged into the boostedengine, reverse flow into the reservoir can be reduced while alsoallowing a pressure of the compressed intake air to be raised higherthan would be otherwise possible. A combination of the high pressuredischarge from the reservoir followed by high pressure air from thecompressor allows a torque demand at tip-in to be better met while alsoexpediting turbine spin-up and reducing turbo lag.

In some embodiments, holding the throttle valve closed for the durationcan lead to compressor surge issues when the throttle is subsequentlyopened. If the boost operation at throttle opening is surge limited, thecontroller may open a compressor relief valve while opening the throttleto address the compressor surge.

Boost reservoir 54 may also be charged with air drawn from the intakemanifold, downstream of compressor 14 and charge air cooler 18. Morespecifically, the boost reservoir 54 is configured to be charged withcompressed intake air from the intake manifold, drawn from downstream ofcompressor 14 and upstream of intake throttle valve 20, via boostreservoir intake charge valve 82. The boost reservoir intake chargevalve 82 may be a normally closed valve commanded to open when a flow ofpressurized intake aircharge from the intake manifold to the boostreservoir is desired. In one example, during low boost conditions, theintake charge valve may be opened to drive at least some intake airpressurized by the compressor into boost reservoir 54. As anotherexample, during high boost conditions, the intake charge valve may beopened to drive some compressed intake air into boost reservoir 54wherein it is mixed with pre-stored combusted exhaust gas to generatehigh pressure EGR. Then, during boosted conditions when a transient EGRrequest is received, the high pressure EGR is discharged into the intakemanifold via intake discharge valve 84 to provide the requested highpressure EGR. A check valve 92 coupled upstream of intake charge valve82 allows compressed air from the compressor to flow into the boostreservoir under conditions of high throttle-inlet pressure (TIP) and tobe stored therein, but it prevents stored compressed air from flowingback to the compressor under conditions of low TIP.

Boost reservoir 54 is also shown selectably coupled to exhaust manifold36 upstream of turbine 16. More specifically, the boost reservoir 54 isconfigured to discharge pressurized charge to the exhaust manifold,upstream of turbine 16, via boost reservoir exhaust discharge valve 88.The boost reservoir exhaust discharge valve 88 may be a normally closedvalve commanded to open when a flow of charge from the boost reservoirto the exhaust manifold is desired. Check valve 98 may be coupleddownstream of the exhaust discharge valve and oriented to prevent thebackward flow of the pressurized charge into the boost reservoir. Inother embodiments, the check valve may be omitted and other measurestaken to prevent backwards flow to the reservoir.

Boost reservoir 54 may also be charged with combusted exhaust gasesdrawn from the exhaust manifold, upstream of turbine 16. Morespecifically, the boost reservoir 54 is configured to be charged withcombusted exhaust gases drawn from the exhaust manifold, upstream ofturbine 16, via boost reservoir exhaust charge valve 86. The boostreservoir exhaust charge valve 86 may be a normally closed valvecommanded to open when a flow of combusted exhaust gas from the exhaustmanifold to the boost reservoir is desired. In one example, during lowboost conditions, or low engine speed-load conditions, the exhaustcharge valve may be opened to drive at least some combusted exhaust gasinto boost reservoir 54. In this way, the EGR percentage of the boostreservoir charge may be increased. A check valve 96 coupled upstream ofexhaust charge valve 86 allows combusted exhaust gas from the intakemanifold to flow into the boost reservoir and to be stored therein, butit prevents the exhaust gas from flowing back.

In this way, during a first condition, the boost reservoir may beselectively charged with only intake air from the intake manifold,downstream of a compressor, while during a second condition, the boostreservoir may be selectively charged with only combusted exhaust gasfrom the exhaust manifold, upstream of a turbine.

In fact, the configuration of boost reservoir 54 vis-à-vis the engineintake and exhaust manifolds enables various options for charging anddischarging the boost reservoir. As a first example, such as when theengine system is operated in a first mode, the reservoir may be chargedwith compressed intake air from the intake manifold, and then responsiveto a tip-in (or during high boost conditions), the compressed intake airmay be discharged to the intake manifold to reduce turbo lag and assistin turbine spin-up. As a second example, such as when the engine systemis operated in a second mode, the reservoir may be charged withcompressed intake air from the intake manifold, the compressed intakeair may be discharged to the exhaust manifold to raise exhausttemperatures and assist in turbine spin-up. As a third example, such aswhen the engine system is operated in a third mode, the reservoir may becharged with combusted exhaust gas from the exhaust manifold, and thenduring boosted conditions, when EGR is requested, the combusted exhaustgas may be discharged to the intake manifold to provide the desired EGR.As a fourth example, such as when the engine system is operated in afourth mode, the reservoir may be charged with combusted exhaust gasfrom the exhaust manifold, and then responsive to a tip-in, thecombusted exhaust gas may be discharged to the exhaust manifold to raisethe exhaust pressure upstream of the turbine, and assist in turbinespin-up. In still further examples, the reservoir may be charged with atleast some combusted exhaust gas and at least some compressed intake airto provide a boost charge of a selected composition (e.g., desired EGRpercentage, desired AFR, etc.) and then, at a later time, thepressurized charge may be discharged to either the intake manifold (forexample, to provide EGR) or to the exhaust manifold (for example, toraise the exhaust pressure).

In some embodiments, boost reservoir 54 may also be charged with theeffluent of one or more unfueled cylinders (that is, charged withunfueled and uncombusted exhaust gas). Specifically, when engine 10 isoperated in DFSO mode, where some of the combustion chambers receive nofuel and merely pump the air admitted through their respective intakevalves, the air pumped and thereby compressed by the unfueled combustionchambers may be drawn from exhaust manifold via exhaust charge valve 86and stored in reservoir 54.

In the various engine systems discussed above, and in others fullyconsistent with this disclosure, pressurizing air or an air/exhaustmixture in a boost tank may cause water vapor to condense inside theboost tank. Therefore in some embodiment, a drain valve (not shown) maybe coupled to boost reservoir 54. The drain valve may be opened asneeded by electronic control system 38 to drain condensate from theboost tank onto the road surface below the vehicle in liquid form, ordirected to the exhaust system of the vehicle, evaporated, anddischarged as a vapor.

The configuration of FIG. 1 enables air stored in the boost reservoir tobe discharged in response to at least a tip-in condition, where thethrottle valve opens suddenly and the compressor is spinning too slowlyto provide the desired intake manifold pressure (MAP). As elaboratedherein below, during at least some tip-in conditions (such as when theboost level at tip-in is lower and anticipated turbo lag is higher),while discharging air from the boost reservoir, a higher amount of sparkretard may be used to rapidly raise the temperature of exhaust gas andexpedite turbine spin-up. During other tip-in conditions (such as whenthe boost level at tip-in is higher and anticipated turbo lag is lower),while discharging air from the boost reservoir, a smaller amount ofspark retard (e.g., no spark retard) may be used to provide additionalengine torque (corresponding to the discharged amount of boost air) tomeet the torque demand while the compressor reaches the desiredcapacity.

In some embodiments, at least some cylinders of the engine may beconfigured to have spark timing retarded while boost air is dischargedinto the intake manifold for purposes of heating exhaust gas andexpediting turbine spin. At the same time, other cylinders may beconfigured to maintain ignition timing while boost air is discharged forthe purposes of torque generation. To reduce potential issues arisingfrom a torque differential between the cylinders, the cylinders enablingexhaust gas heating and the cylinder enabling torque generation may beselected based their firing order. In this way, by expediting turbinespin-up, while providing torque, turbo lag can be reduced whileincreasing net engine combustion torque.

The configurations described above enable various methods for providingcharge including air and/or combusted exhaust gas to a combustionchamber of an engine or for spinning up a turbine. Accordingly, somesuch methods are now described, by way of example, with continuedreference to the above configuration. It will be understood, however,that the methods here described, and others fully within the scope ofthis disclosure, may be enabled via other configurations as well. Themethods presented herein include various measuring and/or sensing eventsenacted via one or more sensors disposed in the engine system. Themethods also include various computation, comparison, anddecision-making events, which may be enacted in an electronic controlsystem operatively coupled to the sensors. The methods further includevarious hardware-actuating events, which the electronic control systemmay command selectively, in response to the decision-making events.

Now turning to FIG. 2, an example routine 200 is shown for charging theboost reservoir of FIG. 1. By charging the boost reservoir withcombusted exhaust gas from the exhaust manifold, exhaust energy may bepre-stored in the reservoir and discharged at a later time to eitherprovide EGR (when discharged into the intake manifold) or raise exhaustpressure (when discharged into the exhaust manifold). By charging theboost reservoir with pressurized intake air from the intake manifold,boost energy may be pre-stored in the reservoir and discharged at alater time to either provide extra boost (when discharged into theintake manifold) or raise exhaust pressure (when discharged into theexhaust manifold). In particular, turbine energy can be increased byincreasing the pre-turbine exhaust pressure. In each case, by storingcharge in the boost reservoir for use at a later time, boosted engineperformance can be improved.

At 202, routine 200 includes estimating and/or inferring engineoperating conditions. These may include, for example, engine speed,torque demand, boost demand, exhaust temperature, barometric pressure,boost reservoir conditions, etc.

In one example, boost reservoir conditions may be estimated using one ormore sensors coupled to the reservoir, such as pressure, temperature,and air-fuel ratio sensors. However, in other examples, one or moreboost reservoir conditions may be inferred or retrieved from a memory ofthe controller rather than being sensed per se. For example, where theboost reservoir was previously charged using air from the intakemanifold, based on compressor conditions, intake air temperature andpressure conditions, as well as EGR demands at the time of charging, astate of the charge in the boost reservoir may be inferred. As anotherexample, where the boost reservoir was previously charged with combustedexhaust gas from the exhaust manifold, based on engine operatingconditions, exhaust conditions, and EGR demands at the time of charging,a state of the charge in the boost reservoir may be inferred. Likewise,where the boost reservoir was previously discharged to the intakemanifold, based on the duration of discharging as well as boostconditions during the discharging, a state of charge (if any) remainingin the boost reservoir may be inferred. In the same way, where the boostreservoir was previously discharged to the exhaust manifold, based onthe duration of discharging as well as engine conditions during thedischarging, a state of charge (if any) remaining in the boost reservoirmay be inferred.

At 204, based on the estimated conditions, it may be determined if aboost reservoir charging opportunity is present. In one example,reservoir charging conditions may be present if the boost reservoir issufficiently empty (e.g., boost reservoir pressure being lower than athreshold). As another example, reservoir charging conditions may bepresent if the engine is operating at a sufficiently high boost level(e.g., operating with boost at higher than a threshold level). As yetanother example, reservoir charging conditions may be confirmed duringan engine DFSO operation. As still another example, reservoir chargingconditions may be confirmed during a transient following a tip-outevent. As such, based on engine operating conditions at the time thecharging opportunity is confirmed, it may be determined whether thecharge the boost reservoir with compressed air from the intake manifoldand/or combusted exhaust gas from the exhaust manifold. For example, aselaborated below, the boost reservoir may be selectively charged basedon engine speed, vehicle speed, manifold pressure, etc. at the time ofthe charging opportunity.

If charging conditions are confirmed, then at 206, the boost reservoirmay be charged with one or more of compressed intake air from the intakemanifold and combusted exhaust gas from the exhaust manifold.Specifically, the reservoir intake charge valve may be opened for aduration to charge the reservoir with compressed intake air from theintake manifold, and/or the reservoir exhaust charge valve may be openedfor a duration to charge the reservoir with exhaust gas from the exhaustmanifold. A duration of opening of the intake charge valve and/or theexhaust charge valve may be adjusted to adjust the composition of chargestored in the reservoir so as to provide a desired boost reservoircharge EGR percentage (or dilution). In one example, the boost reservoirmay be charged with air and combusted exhaust gas to provide charge of adesired EGR percentage and desired pressure, such that when thepressurized charge is eventually discharged during a subsequent boostedengine operation, high pressure EGR can be enabled.

For example, during a first condition, when a tip-in is predicted athigh engine speeds, the boost reservoir may be charged with combustedexhaust gases only. Herein, the engine may be operating at higher enginespeeds with a pedal position near a closed position and with a vehiclespeed being higher than a threshold speed but with an exhaust pressurebeing greater than a threshold pressure. In comparison, during a secondcondition, when a tip-in is predicted at low engine speeds, the boostreservoir may be charged with fresh intake air and combusted exhaustgases, with a ratio of the fresh intake air to combusted exhaust gasesadjusted based on a desired boost reservoir EGR percentage.Alternatively, during the second condition, the boost reservoir may becharged with fresh intake air only. Herein, the engine may be operatingat lower engine speeds with the pedal position near a closed positionand with a vehicle speed being lower than a threshold speed but with anintake manifold pressure being greater than a threshold pressure. Forexample, the engine may be operating with positive intake to exhaustmanifold pressure.

As elaborated at FIG. 5, during some conditions, the boost reservoir maybe charged with a first amount of combusted exhaust gas at a first,lower pressure from the exhaust manifold, upstream of the turbine. Thisinitial charging increases the EGR percentage of the reservoir chargebut the stored exhaust gas is at a lower pressure. To further raise thepressure of the stored charge, the boost reservoir may be subsequentlyfurther charged with a second amount of fresh intake air at a second,higher pressure from the intake manifold, downstream of the compressor.This later charging slightly decreases the EGR percentage of thereservoir charge but raises the charge pressure. The first and secondamounts may be adjusted to provide a desired EGR percentage of thepressurized charge. The stored charge can then be advantageouslydischarged during selected boosted engine conditions to provide highpressure EGR benefits.

As another example, the boost reservoir may be charged with at leastsome combusted exhaust gases (e.g., with only combusted exhaust gases)during a tip-out at lower engine speeds. In comparison, during a tip-outat higher engine speeds, the controller may charge the boost reservoirwith at least some compressed intake air from the intake manifold (e.g.,with only compressed intake air). As yet another example, when chargingconditions are confirmed during an engine DFSO operation, the reservoirmay be charged with uncombusted exhaust gas released from the cylindershaving fuel shut-off.

As such, following the charging, boost reservoir conditions may beupdated in the controller's memory. In one example, boost reservoirconditions may be updated using one or more sensors coupled to thereservoir, such as pressure, temperature, and air-fuel ratio sensors.However, in other examples, boost reservoir conditions may be inferredand updated in the memory of the controller rather than being sensed perse. For example, where the boost reservoir was recently charged usingair from the intake manifold, based on compressor conditions, intake airtemperature and pressure conditions, as well as EGR demands at the timeof charging, a state of the charge in the boost reservoir may beinferred and updated. As another example, where the boost reservoir wascurrently charged with combusted exhaust gas from the exhaust manifold,based on engine operating conditions, exhaust conditions, and EGRdemands at the time of charging, a state of the charge in the boostreservoir may be inferred and updated.

In one example, the EGR percentage of the boost reservoir may beestimated or inferred based on one or more an exhaust air-fuel ratiosensor output, MAF, and a fuel injector pulse-width. The controller maybe configured to estimate a volume of gas that was stored in thereservoir based on a boost reservoir pressure. The controller may thenestimate how much of that volume was air based on MAF changes followingdischarging of the pressurized charge, and how much of that volumeincluded fuel based on fuel injection adjustments following dischargingof the pressurized charge (e.g., based on a fuel injector pulse-width).An air-to-fuel ratio estimated may then be based on the air and fuelestimates. In an alternate example, the estimated air-to-fuel ratio maybe based on the output of an intake oxygen sensor. The estimatedair-to-fuel ratio may then be compared to a measured air-to-fuel ratioto map an error. The error may then be used to update an EGR percentageestimate of the boost reservoir charge. The stored boost reservoirconditions may be retrieved by the controller during a subsequentdischarging operation. It will be appreciated that in all cases, thecharging may be performed during an engine cycle preceding a tip-inevent where the pressurized charge is discharged.

In this way, a boost reservoir may be selectively charged with one ormore of fresh intake air from an intake manifold and combusted exhaustgas from an exhaust manifold. The charging with fresh intake air andcombusted exhaust gas may be performed to enable storing of a boostreservoir charge having a selected EGR percentage. As elaborated hereinwith reference to FIG. 3, following the selective charging, such as inresponse to a tip-in, the pressurized charge may be discharged from theboost reservoir to the intake manifold and/or the exhaust manifold,based on engine operating conditions at the time of the tip-in, tothereby reduce turbo lag and improve boosted engine performance.

Now turning to FIG. 3, an example routine 300 is shown for dischargingthe boost reservoir of FIG. 1. By discharging the boost reservoir intothe intake manifold or the exhaust manifold, based at least on acomposition of charge in the boost reservoir, boost charge may beadvantageously used to raise exhaust temperature or pressure. In eachcase, by discharging from the boost reservoir in response to a tip-in,turbo lag can be reduced and boosted engine performance can be improved.

At 302, engine operating conditions may be estimated and/or inferred.These may include, for example, engine speed, torque demand, boostdemand, exhaust temperature, barometric pressure, boost reservoirconditions, etc. At 304, boost reservoir charge details may beretrieved. As such, the pressurized charge may include a variablemixture of combusted exhaust gas and compressed intake air, therebyhaving a distinct charge pressure and charge EGR percentage (ordilution). The retrieved details may include, for example, a chargecomposition including a fresh air content of the charge as well as acombusted exhaust gas content of the charge. The retrieved details mayfurther include charge temperature, charge pressure, charge EGRpercentage, etc. As previously elaborated, the boost reservoir detailsmay be stored in the controller's memory and may be inferred and updatedfollowing each charging operation. In addition, following anydischarging operation, the boost reservoir details may be updated toreflect the most recent state of the charge remaining (if any) in theboost reservoir.

At 306, a tip-in may be confirmed. In one example, a tip-in may beconfirmed in response to an accelerator pedal being displaced beyond athreshold position and a torque demand being higher than a threshold. Ifa tip-in is not confirmed, the routine may end. Upon confirming thetip-in, at 308, the routine includes determining whether to dischargethe pressurized charge to the intake or the exhaust manifold.

In one example, the selection (of whether to discharge to the intake orthe exhaust manifold) may be based on a composition (or EGR percentage)of charge stored in the boost reservoir. For example, when the boostreservoir has a high fresh air content (e.g., when the fresh airpercentage of the stored charge is higher than a threshold amount) or alow EGR content (e.g., when the EGR percentage of the stored charge islower than a threshold amount), the boost air may be applied to theintake manifold to provide increased torque to address turbo lag whilethe turbine spools up. As another example, when the boost reservoir hasa low fresh air content (e.g., when the fresh air percentage of thestored charge is lower than the threshold amount) or a high EGR content(e.g., when the EGR percentage of the stored charge is higher than thethreshold amount), the boost air may be applied to the exhaust manifoldto enable energy from the boost charge pressure to be extracted andadvantageously applied to expedite turbine spool-up. Thus, during afirst tip-in, when the discharged charge has a lower EGR percentage, thedischarging is performed into the intake manifold, while during a secondtip-in, when the discharged charge has a higher EGR percentage, thedischarging is performed into the exhaust manifold.

In still another example, the selection of whether to discharge theboost air to the intake manifold or the exhaust manifold may be furtherbased on a charge pressure of the pressurized charge stored in thereservoir. For example, when the boost reservoir charge pressure ishigher than a threshold pressure, the higher pressure charge may beselectively discharged to the intake manifold to rapidly raise exhausttemperatures and reduce turbo lag. In an alternate example, when theboost reservoir charge pressure is lower than the threshold pressure,the lower pressure charge may be selectively discharged to the exhaustmanifold to rapidly raise exhaust pressure and reduce turbo lag.

In a further example, the selection may be based on a boost level at thetime of tip-in. For example, when the boost level at the time of tip-inis higher than a threshold boost level, the boost reservoir charge maybe discharged into the intake manifold. In comparison, when the boostlevel at the time of tip-in is lower than the threshold level, the boostcharge may be discharged into the exhaust manifold. In an alternateexample, an engine boost level at a first tip-in where the pressurizedcharge is discharged to the intake manifold may be lower than the engineboost level at a second tip-in where the pressurized charge isdischarged to the exhaust manifold. In still further embodiments, theselection of whether to discharge the boost charge to the intake orexhaust manifold may be based on other engine operating conditions, suchas engine speed, exhaust temperature, and exhaust air-to-fuel ratio.

As yet another example, the selection may be further based on an EGRdemand at the time of tip-in. For example, the boost reservoir may becharged with combusted exhaust gas and compressed intake air to storecharge of a defined charge pressure and defined charge EGR percentage.Then, when the boost level at a tip-in is lower than the boost reservoircharge pressure, the pressurized charge may be discharged to the intakemanifold if EGR is requested, and discharged to the exhaust manifold ifEGR is not requested. In comparison, when the boost level at tip-in ishigher than the boost reservoir charge pressure, the pressurized chargemay be discharged to the exhaust manifold only.

It will be appreciated that while the depicted routine suggestsdischarging to either the intake manifold or the exhaust manifold duringa tip-in, in some embodiments, the pressurized charge may be dischargedto each of the intake manifold and the exhaust manifold during a giventip-in. Specifically, in those embodiments, the pressurized charge maybe sequentially discharged to each of the intake and the exhaustmanifold during the same tip-in. Accordingly, prior to discharging, itmay be determined whether the pressurized charge is to be discharged tothe intake manifold first followed by the exhaust manifold, or whetherthe pressurized charge is to be discharged to the exhaust manifold firstfollowed by the intake manifold. In one example, the order of sequentialdischarging may be based on boost pressure, exhaust pressure, and boosttank pressure, for example.

At 310, it may be confirmed if the boost reservoir charge is to bedischarged to the intake manifold (e.g., if the boost reservoir chargeis to be discharged only to the intake manifold, or initially to theintake manifold). If yes, then at 312, the routine includes dischargingpressurized charge from the boost reservoir to the intake manifold.Specifically, to the intake manifold, downstream of a turbochargercompressor and downstream of an intake throttle. In addition, during thedischarging, spark timing may be retarded based on the amount ofpressurized charge discharged from the reservoir. However, the appliedspark retard may be less than a spark retard limit based on a combustiontorque corresponding to the discharged amount of pressurized air. Thatis, spark may not be retarded beyond an amount that reduces the netcombustion torque. For example, the spark retard may maintain orincrease torque above the torque level generated during cylinderoperation in the absence of supplementary pressurized air dischargedfrom the boost reservoir. This allows a net combustion torque of theengine to be increased, or at least maintained, during the retarding ofignition timing.

In one example, the discharging to the intake manifold may occur outsideof a valve overlap period. For example, the discharging may occur duringan intake stroke and/or a compression stroke, but not during portions ofthese strokes in which both the intake and exhaust valve of the cylinderare concurrently open. As such, this allows the air-fuel mixture to becombusted in the cylinder such that upon release, the heated exhaust gascan be used to spool the turbine on a subsequent combustion event. Bydischarging the pressurized gas outside of the overlap period, ratherthan within the overlap period, more air-fuel mixing can be achieved andbetter exhaust gas heating may be achieved. However, in an alternateexample, the discharging to the intake manifold may occur during a valveoverlap period. For example, a timing of the discharging may be adjustedto coincide with positive valve overlap. Alternatively, a cam timing ofa variable cam timing mechanism may be adjusted based on the dischargingto provide high valve overlap when the boost reservoir is discharged.Then, following the discharging, the cam timing of the variable camtiming mechanism may be reset based on engine operating conditions.

In one embodiment, as elaborated with reference to FIG. 4, a portion ofthe pressurized charge may be discharged from the boost reservoir to theintake manifold, downstream of an intake throttle, while holding theintake throttle closed (or while adjusting a position of the throttletowards a more closed position). Then, a remaining portion of thepressurized charge may be discharged after opening the throttle. As alsoelaborated in the example of FIG. 8, the throttle may be held closeduntil a threshold throttle inlet pressure is generated upstream of thethrottle by the compressor. By holding the throttle closed, a boostpressure generated at the compressor may be raised more rapidly thanwould be otherwise possible with the throttle open. At the same time,torque demand may be met, and turbine spool-up may be expedited bydischarging pressurized charge into the intake manifold.

At 314, during the discharging, an amount of exhaust gas recirculatedfrom the exhaust manifold to the intake manifold may be reduced. Inparticular, the reducing of EGR may be based on the discharged amount ofpressurized air. This allows the combustion stability to be improved andincreased spark retard to be used for heating exhaust gas. In oneexample, where the engine system has an EGR passage including an EGRvalve for recirculating an amount of exhaust gas from the engine exhaustmanifold to the engine intake manifold, an engine controller may reducean opening of the EGR valve to reduce the amount of exhaust gasrecirculated to the engine intake via the EGR passage.

At 316, it may be determined if a temperature of the exhaust gas (Texh)is higher than a threshold. Herein, the threshold exhaust gastemperature may correspond to a temperature above which the turbine canbe spooled and spun-up so as to drive the compressor and provide adesired boost. For example, the threshold temperature may be based on aturbine speed. Thus, if the exhaust temperature is above the thresholdtemperature, at 318, the discharging of pressurized charge from theboost reservoir to the intake manifold may be discontinued.Additionally, at 330, the turbine may be spooled and the turbochargercompressor may be operated to provide the required amount of boost tomeet the torque demand. If the threshold exhaust temperature has notbeen attained at 316, the discharging of pressurized charge to theintake manifold, while retarding spark, is continued until the exhausttemperature is above the threshold temperature.

Returning to 310, if discharging of the boost reservoir charge to theintake manifold is not confirmed, then at 320, it may be confirmed ifthe boost reservoir charge is to be discharged to the exhaust manifold(e.g., if the boost reservoir charge is to be discharged only to theexhaust manifold, or initially to the exhaust manifold). If yes, then at322, the routine includes discharging pressurized charge from the boostreservoir to the exhaust manifold while adjusting a cylinder fuelinjection (including a fuel injection amount and/or timing) during thedischarging based on the discharged pressurized charge so as to maintainan overall exhaust air-to-fuel ratio (e.g., an exhaust air-to-fuel ratiosensed at an exhaust catalyst) at or around stoichiometry. As usedherein, discharging pressurized charge to the exhaust manifold includesdischarging the pressurized charge to the exhaust manifold upstream of aturbocharger turbine. In one example, the discharging may be performedduring boosted engine operation.

In some embodiments, a simultaneous throttle adjustment may be performedto compensate for the increased exhaust pressure reducing the amount ofair that can be inducted into the engine intake, and therefore theamount of torque delivered. For example, an opening of the throttle maybe simultaneously increased to increase air inducted and torque outputfrom the engine.

Adjusting the cylinder fuel injection while discharging to the exhaustmanifold may include, for example, performing a rich fuel injectionand/or a late fuel injection based on an amount and an air-to-fuel ratioof the pressurized charge. By retarding and/or enriching the fuelinjection so as to match the (fresh) air component of the chargedissipated from the boost tank into the exhaust manifold, an overallmixture at the exhaust (e.g., at a downstream exhaust catalyst) may bemaintained substantially at stoichiometry. Further, the exothermicreaction of the extra oxygen from the air in the boost reservoir withthe rich fuel injection generates additional exhaust heat and exhaustpressure which also helps to reduce turbo lag. In one example, the richfuel injection may be performed when the exhaust temperature is higherthan a threshold temperature to better ensure that the exothermicreaction will occur in the exhaust manifold, as desired, and not furtherdownstream. In one example, the fuel injection may be adjusted usingfeedback from one or more air-to-fuel ratio sensors, such as fromair-to-fuel ratio sensors located upstream and/or downstream of theturbine and the exhaust catalyst in the exhaust manifold. As such,air-fuel mixing may be an issue if relying on feedback from an upstreamair-to-fuel ratio sensor. Thus, in some embodiments, to enable morereliable feedback signals to be received, during the discharging, thecylinder fuel injection may be adjusted based on feedback from anexhaust air-to-fuel ratio sensor located downstream of the turbochargerturbine in the exhaust manifold.

Next, at 324, it may be determined if a pressure of the exhaust gas(Pexh), upstream of the turbine, is higher than a threshold. Herein, thethreshold exhaust gas pressure may correspond to a pressure above whichthe turbine can be spooled and spun-up so as to drive the compressor andprovide a desired boost. For example, the threshold pressure may bebased on a turbine speed. Thus, if the exhaust pressure is above thethreshold pressure, at 328, the discharging of pressurized charge fromthe boost reservoir to the exhaust manifold may be discontinued.Additionally, at 330, the turbine may be spooled and the turbochargercompressor may be operated to provide the required amount of boost tomeet the torque demand. If the threshold exhaust pressure has not beenattained at 324, the discharging of pressurized charge to the exhaustmanifold, while retarding and/or enriching cylinder fuel injection, iscontinued until the exhaust pressure is above the threshold pressure.

It will be appreciated that while the depicted routine illustratesdischarging to the exhaust manifold until a threshold exhaust pressureis achieved, in alternate examples, the controller may be configured tocontinue discharging to the exhaust manifold until the turbine speedreaches a threshold speed or until an intake boost pressure (e.g., atthe compressor) reaches a threshold boost pressure. For example, thedischarging to the exhaust manifold may be performed during a positiveintake manifold to exhaust manifold pressure condition. Herein, when theintake manifold pressure reaches a threshold pressure, and the positiveintake to exhaust manifold pressure condition ceases to exist, thedischarging to the exhaust manifold is discontinued. That is, an enginecontroller may be configured to discharge to the exhaust manifold untilturbine speed reaches a threshold speed or until intake boost pressurereaches a threshold boost pressure.

In this way, a boost reservoir may be charged with at least somecombusted exhaust gases from the exhaust manifold during an engine cyclepreceding a tip-in. Then, in response to a tip-in, turbo lag may bereduced by discharging pressurized charge from the boost reservoir tothe exhaust manifold. An example engine operation with discharging of aboost reservoir charge to an exhaust manifold is elaborated herein withreference to FIG. 6.

It will be appreciated that in some examples, the pressurized charge maybe discharged to each of the intake manifold and the exhaust manifoldduring the same tip-in. Specifically, on a single tip-in, a portion(e.g., a first amount) of the pressurized charge stored in the boostreservoir may be discharged to the exhaust manifold, while a remainingportion (e.g., a second, different amount) of the stored charge isfurther discharged to the intake manifold. Herein, the controller maydecide whether to discharge to the intake manifold first or the exhaustmanifold first based on the same considerations discussed above. Thus,in one example, when the boost reservoir pressure is higher, a portionof the pressurized charge may be discharged to the intake manifold firstand then a remaining portion may be discharged to the exhaust manifoldlater. In an alternate example, when the pressurized charge has a higherEGR content, a portion of the pressurized charge may be discharged tothe exhaust manifold first and then a remaining portion may bedischarged to the intake manifold later. By discharging to each of theintake and the exhaust during the same tip-in, it is possible to betterbalance the competing objectives of air/fuel mixing, fast spool up, andsufficient duration of increased output from the boost tank to fillsubstantially the entire turbo lag delay.

It will be appreciated that a tip-out following the tip-in may providean opportunity for recharging the boost reservoir. For example, duringthe tip-out, the engine controller may selectively charge the boostreservoir with intake air from the intake manifold or combusted exhaustgas from the exhaust manifold, the selection based on a reservoircomposition at the tip-out. The selection may be further based on anengine speed and a vehicle speed at the time of tip-out, as previouslydiscussed at FIG. 2.

In this way, during a first tip-in, pressurized charge is dischargedfrom a boost reservoir to an intake manifold while during a secondtip-in, pressurized charge is discharged from the boost reservoir to anexhaust manifold. By discharging pressurized charge from the boostreservoir to the intake manifold during some conditions and to theexhaust manifold during other conditions, benefits from the use of apressurized charge stored in a boost reservoir can be extended.Specifically, pressurized fresh intake air can be better used to reduceturbo lag while also meeting an interim torque demand. Likewise,pressurized exhaust gas can be better used to reduce turbo lag whilealso meeting EGR demands. Overall, boosted engine performance can beimproved.

Now turning to FIG. 4, an example routine 400 is shown for dischargingpressurized charge from a boost reservoir to an intake manifold whilecontrolling an intake throttle. By delivering a portion of thepressurized charge to the intake manifold with the intake throttleclosed, a boost pressure, or throttle inlet pressure, may be rapidlyraised, allowing the compressor boost to be “pre-charged” beforedelivery into the intake manifold. In the mean time, torque demand maybe met by discharging pressurized charge from the boost reservoirdownstream of the throttle. In one example, the routine of FIG. 4 may beperformed as part of the routine of FIG. 3, such as at step 312.

At 402, routine 400 includes confirming that the pressurized charge fromthe boost reservoir is to be discharged to the intake manifold. If not,the routine may end. In one example, pressurized charge may bedischarged to the intake in response to a tip-in during boosted engineoperation. Herein, during the tip-in, a throttle inlet pressure (TIP),estimated upstream of an intake throttle, may be below a threshold. Assuch, the boost reservoir may have been charged during an engine cyclepreceding the tip-in with one or more of compressed air from the intakemanifold or combusted exhaust gas from the exhaust manifold to storepressurized charge having a defined EGR percentage and a defined chargepressure.

Upon confirmation, at 404, the routine includes opening the boostreservoir intake discharge valve while closing the air intake throttle.The controller may then discharge the pressurized charge from the boostreservoir into the intake manifold, downstream of the intake throttle,while holding the intake throttle closed. Discharging pressurized chargeto the intake manifold may include opening the reservoir intakedischarge valve while maintaining the reservoir intake charge valveclosed.

At 406, the routine includes operating the compressor while monitoring athrottle inlet pressure, TIP, (also indicative of a boost pressuregenerated at the compressor while the throttle is held closed). In oneexample, TIP may be estimated by a pressure sensor position in theintake manifold downstream of the compressor and upstream of the airintake throttle. As such, while the turbine gradually spools up, thecompressor pressure also gradually increases. Accordingly, TIP alsoincreases. Herein, by holding the throttle closed, the increase in boostpressure, or TIP, may be expedited. As a result, by the time thethrottle is opened, a sufficient amount of pressurized boost air may begenerated by the compressor and stored, or pre-charged, upstream of thethrottle. This boost pressure can then be delivered to the intakemanifold as soon as the throttle is opened.

Thus, the discharging of pressurized charge to the intake manifold whileholding the intake throttle closed may be continued for a duration untilthe throttle inlet pressure is at or above a threshold. Alternatively,the discharging with the throttle held closed may be continued for aduration until a manifold pressure (MAP) downstream of the throttlematches the throttle inlet pressure upstream of the throttle.

At 408, it may be determined if the estimated TIP is higher than thethreshold. In one example, the threshold may be based on a desired boostlevel or boost pressure. If TIP has reached the desired boost levelbefore the boost reservoir has been completely discharged (or at thesame time as the boost reservoir is fully discharged), then at 412,after the duration has elapsed, the routine includes closing the intakedischarge valve while opening the throttle from the closed position. Dueto opening of the intake throttle, pressurized aircharge that was storedand pre-charged upstream of the throttle may be discharged fromdownstream of the compressor into the intake manifold. That is,compressed air may be directed from the compressor into the intakemanifold with the throttle open. As such, directing compressed air intothe intake manifold with the throttle open includes discontinuing thedischarging of pressurized charge to the intake manifold. Therein, eachof the reservoir intake charge valve and discharge valve may bemaintained closed.

As such, the boost reservoir may be discharged even before TIP reachesthe threshold. Thus, if TIP is not above the threshold, then at 410, itmay be determined if the boost reservoir has been completely discharged.If yes, then also the routine proceeds to 412 to close the intakedischarge valve and open the throttle to allow aircharge pressurized atthe compressor to be discharged into the intake manifold.

The controller may be further configured to adjust a spark timing whiledischarging the pressurized charge to the intake manifold and whiledirecting compressed air to the intake manifold. For example, sparktiming may be adjusted to a first timing during the discharging whileholding the intake throttle closed while spark timing is adjusted to asecond, different timing while directing compressed air with thethrottle open. Herein, the first timing may be based on an amount andthe EGR percentage of the discharged pressurized charge. Likewise, thesecond timing may be based on an amount and pressure of compressed airdirected to the intake manifold. The second timing may also be based onthe EGR percentage of the directed compressed air if any EGR (e.g., highpressure EGR or low pressure EGR via respective EGR passages) isperformed while directing compressed air from the compressor to theintake manifold.

In this way, during a tip-in, while operating a compressor, cylinderpressure may be raised by discharging pressurized charge from a boostreservoir to an intake manifold while holding an intake throttle closed.The cylinder pressure may then be further raised during the tip-in bydirecting compressed air from the compressor to the intake manifoldwhile opening the closed throttle. By delivering pressurized charge froma boost reservoir with the intake throttle closed, the discharged chargemay be used to expedite turbine spool-up and reduce turbo lag while alsomeeting the engine torque demand during the turbo lag. By holding theintake throttle closed for a duration while the turbine spools up, apressure of air compressed by the compressor may be rapidly raised. Inaddition, higher boost pressures may be attained. By then delivering thepressurized boost charge to the intake manifold after the throttle hasbeen opened, boost benefits can be achieved. Overall, turbo lag isreduced while boost performance is enhanced. An example engine operationwith discharging of pressurized charge from a boost reservoir to theintake manifold with the throttle closed for a duration is elaboratedherein with reference to FIG. 8.

Now turning to FIG. 5, an example routine 500 is described for charginga boost reservoir with combusted exhaust gases and pressurized fresh airto generate a pressurized EGR mixture which can then be discharged tothe intake manifold during boosted conditions to enable high pressureEGR benefits to be achieved.

At 502, engine operating conditions may be estimated and/or inferred. At504, it may be confirmed that the engine is operating with boost butwith a boost level that is lower than a threshold level. In one example,it may be confirmed that the boost level is above a lower threshold butbelow an upper threshold. If not, the routine may end. Uponconfirmation, at 506, the routine includes, during low boost conditions,charging the boost reservoir with at least some combusted exhaust gas toa first, lower pressure. For example, the boost reservoir may be chargedwith only combusted exhaust gases from the exhaust manifold by openingthe boost reservoir exhaust charge valve (for a duration).Alternatively, the reservoir may be charged with combusted exhaust gasfrom the exhaust manifold and fresh intake air from the intake manifold.As used herein, charging with combusted exhaust gas includes chargingwith one or more of low pressure EGR, high pressure EGR, and combustedexhaust gas received directly from the exhaust manifold via a valve.Charging the reservoir with combusted exhaust gas includes selectivelyopening a first valve coupled between the boost reservoir and theexhaust manifold. Following charging, the pressurized charge in theboost reservoir may have a defined EGR percentage (that is, thereservoir may be charged with a ratio of intake air to combusted exhausthas to provide the desired boost reservoir EGR percentage) and may be ata first, lower pressure.

At 508, after the charging of the boost reservoir has been completed, itmay be determined if increased boost is required. In one example,increased boost may be demanded in response to a further tip-in whilethe engine is already boosted. In response to the tip-in, at 510, boostmay be increased. For example, a compressor speed may be increased. At512, it may be confirmed that boost has been increased and the boostlevel is now higher than the threshold. Upon confirmation, at 514,during higher boost conditions, the boost reservoir may be furthercharged with compressed intake gas to raise a pressure of the chargestored in the reservoir to a second, higher pressure. The furthercharging with compressed intake gas includes selectively opening asecond valve coupled between the boost reservoir and the intakemanifold, wherein the second valve is coupled between the boostreservoir and the intake manifold downstream of the intake throttle. Bymixing the combusted exhaust gases stored in the reservoir at lowerpressure with the compressed intake air at higher pressure, a highpressure EGR mixture may be generated in situ and stored in the boostreservoir, for subsequent discharging when high pressure EGR isrequired. In another example, the valve may be coupled upstream of thethrottle.

At 516, it may be determined if a transient increase in EGR isrequested. In one example, the transient increase in EGR may berequested at a later time during engine operation when the boost levelis lower than the threshold (e.g., lower than the second pressure) andthe operator tips-in. For example, while the engine is boosted, a tip-intowards wide open throttle may be received. As a result, a transientincrease in EGR may be required. In response to the transient increasein requested EGR, at 518, the routine includes discharging pressurizedcharge from the boost reservoir to an engine manifold (e.g., dischargingto an intake or exhaust manifold). For example, discharging to theintake manifold includes discharging downstream of a turbochargercompressor and downstream of an intake throttle. In this way, bydischarging the pre-stored high pressure EGR from the boost reservoir tothe intake manifold in response to a transient request for increasedEGR, high pressure EGR can be availed. Specifically, the controller mayselectively open a third valve coupled between the boost reservoir andthe intake manifold, downstream of the intake throttle (that is, theboost reservoir intake discharge valve) and discharge the pressurizedcharge from the boost reservoir to the intake manifold downstream of theintake throttle. In one embodiment, while discharging the high pressureEGR from the boost reservoir, the intake throttle may be temporarilyheld closed. In one example, the discharging may be continued until theboost pressure equilibrates with the boost reservoir pressure afterwhich the discharging may be discontinued. For example, the dischargingmay be continued until the boost pressure is at the second pressure.

The inventors herein have recognized that during boosted conditions,when transient EGR is required, the requested EGR may not always beavailable as rapidly as required. Specifically, recirculated exhaust gasmay not be immediately available via low pressure EGR due to the slowerresponse time of the low pressure EGR. At the same time, recirculatedexhaust gas may also not be immediately available via a conventionalhigh pressure EGR passage due to the pressure difference between theintake and exhaust manifolds which would cause the high pressure EGR toflow backwards into the exhaust manifold. To overcome these issues andstill enable high pressure EGR benefits to be achieved, a pressure ofthe exhaust gas in the reservoir may be raised by mixing with an amountof compressed intake air before discharging. This allows high pressureEGR to be provided in response to a tip-in even when boost levels arealready high. An example engine operation with charging of a boostreservoir with high pressure EGR and delivery of the high pressure EGRto the intake manifold during a transient EGR request is elaboratedherein with reference to FIG. 7.

Now turning to FIG. 6, map 600 shows an example engine operation whereinturbo lag is reduced by discharging pressurized charge to an exhaustmanifold in response to a tip-in. Specifically, map 600 depicts a changein pedal position (PP) at plot 602, a change in boost pressure at plot604, an opening or closing state of a boost reservoir exhaust dischargevalve (reservoir valve_exh) at plot 606, a change in exhaust pressure atplot 608, and a change in cylinder air-to-fuel ratio (cylinder AFR)relative to stoichiometry at plot 612. In one example, the boostpressure may be estimated by a pressure sensor positioned in the intakemanifold downstream of a turbocharger compressor, the exhaust pressuremay be estimated by a pressure sensor in the exhaust manifold upstreamof the turbine, and the cylinder air-to-fuel ratio may be estimated byan air-to-fuel ratio sensor coupled to an exhaust catalyst in theexhaust manifold.

Before t1, the engine may be operating with low boost pressure. Forexample, the engine may be operating un-boosted or at a low boost level.At t1, a tip-in event is confirmed, as indicated by the change in pedalposition (plot 602). In response to the tip-in, a controller may beconfigured to discharge charge including air and combusted exhaust gasfrom a boost reservoir into the exhaust manifold, upstream of a turbine,to reduce turbo lag. Specifically, a boost reservoir exhaust dischargevalve may be opened for a duration between t1 and t2 (plot 606).

As elaborated in FIG. 1, the boost reservoir may be coupled to theexhaust manifold via each of a first exhaust charge valve and a secondexhaust discharge valve. Accordingly, discharging charge from thereservoir into the exhaust manifold includes opening the second (exhaustdischarge) valve while maintaining the first (exhaust charge) valveclosed. As such, the boost reservoir may have been charged withcompressed air from the intake manifold and/or combusted exhaust gasfrom the exhaust manifold during a charging opportunity prior to thetip-in. Therein, when charging the reservoir with combusted exhaust gasfrom the exhaust manifold, the second valve may have been opened whilethe first valve was maintained closed. As such, when charging thereservoir with compressed intake air from the intake manifold, areservoir intake charge valve may be have been opened while maintainingan intake discharge valve closed.

In response to the discharging of pressurized charge from the boostreservoir into the exhaust manifold at t1, an exhaust pressure (plot608) may start to increase. Herein, by dissipating pressurized chargefrom the reservoir into the exhaust manifold responsive to the tip-in,an exhaust pressure upstream of the turbine may be raised faster thanwould otherwise be possible. The rapid increase in exhaust pressureenables faster turbine spool-up. This, in turn, allows turbo lag to bereduced and allows a boost pressure at the compressor to be rapidlyraise (plot 604). In comparison, plot 609 (dashed line) shows a slowerincrease in exhaust pressure that may be expected in the absence ofpressurized charge being dissipated from a boost reservoir into theexhaust manifold. Due to the slower increase in exhaust pressure,turbine spool-up may be delayed leading to turbo lag, which is reflectedin the slower rise in boost pressure (at the compressor), as shown atplot 605 (dashed line). It will be appreciated that in both cases, theexhaust pressure is raised to the same level (see plots 608 and 609) andthe boost pressure is also raised to the same level (see plots 604 and605), albeit at different rates. However, by dissipating pressurizedcharge to the exhaust manifold, turbine spinning is expedited, turbo lagis reduced, and boost pressures are rapidly attained. This allowsboosted engine performance to be improved.

During the discharging to the exhaust manifold, the engine controllermay adjust a fuel injection to the engine cylinder to be richer (asshown by enrichment of cylinder AFR at plot 612) and/or later (notshown). Herein, a richness and delay in the fuel injection may be basedon the (discharged) pressurized charge so as to maintain an exhaustair-to-fuel ratio at an exhaust catalyst substantially at or aroundstoichiometry 615. Specifically, the richness of and delay in thecylinder fuel injection may be adjusted to match the fresh compressedair component of the charge dissipated from the boost tank into theexhaust manifold so that an overall mixture sensed at the exhaust (e.g.,downstream of the turbine and downstream of an exhaust catalyst in theexhaust manifold) is maintained substantially at or aroundstoichiometry. In addition, the reaction of the extra oxygen in the aircomponent of the boost reservoir charge with the rich fuel injectiongenerates additional exhaust heat which further assists in expeditingturbine spool-up and reducing turbo lag. For example, the cylinder fuelinjection may be adjusted based on AFR feedback from one or moreair-to-fuel ratio sensors (or oxygen sensors) located downstream of theturbine and the exhaust catalyst in the exhaust manifold. In still otherexamples, the fuel injection may be adjusted based on AFR feedback froman oxygen sensor positioned in the exhaust manifold, upstream of theturbine.

As elaborated above, the degree of richness of the fuel injection (thatis, the cylinder AFR shown on plot 612) may vary based on the amount ofpressurized charge released from the reservoir (or rate of discharge)and the EGR percentage of the discharged pressurized charge. Thus, asthe EGR percentage of the boost reservoir charge decreases (that is,there is a higher ratio of fresh intake air in the charge to combustedexhaust gas), a more rich cylinder fuel injection may be required (asshown by plot 612, solid line). In comparison, as the EGR percentage ofthe boost reservoir charge becomes progressively higher (that is, thereis a progressively ratio of fresh intake air to combusted exhaust gas inthe charge), a richness of the fuel injection may be progressivelydecreased (as shown by plot 613 (dashed line) and plot 614 (dottedline)).

Between t1 and t2, as the pressurized charge from the boost reservoir isdischarged into the exhaust manifold, and as the exhaust pressureupstream of the turbine increases, a rate of discharge from thereservoir into the exhaust manifold decreases. That is, when the exhaustdischarge valve is first opened at t1, pressurized charge may bedischarged into the exhaust manifold at a faster rate. Thus, during thisfaster rate of discharging, when a larger amount of charge is releasedinto the exhaust manifold, the richness of the rich fuel injection maybe relatively higher. Then, as time point t2 approaches, the exhaustpressure (plot 608) starts approaching threshold pressure 610, andpressurized charge may be discharged into the exhaust manifold at aslower rate. Thus, during this slower rate of discharging, when asmaller amount of charge is released into the exhaust manifold, therichness of the rich fuel injection may be relatively lower.Specifically, a gradual tapering down of the richness (and/or delay) ofthe fuel injection occurs, as shown by the cylinder AFR richness gradualtapering down towards stoichiometry 615.

As such, the discharging from the reservoir into the exhaust manifold iscontinued for a duration d1 (between t1 and t2) until an exhaustpressure upstream of the turbine is at threshold pressure 610. At t2,when the exhaust pressure upstream of the turbine is at thresholdpressure 610, the reservoir exhaust discharge valve may be closed (plot606). The threshold pressure 610 may be based on the boost reservoirpressure (not shown). As such, as the reservoir is discharged, theexhaust pressure increases and the boost reservoir pressure decreases.When the exhaust pressure is the same as the boost reservoir pressure,no further flow may be possible, and no further benefits from the boostreservoir charge can be attained. Thus, the threshold pressure 610 mayset based on an expected rate of drop of the boost reservoir pressureand may incorporate a difference so that the exhaust discharge valve isclosed before the exhaust pressure has reached the boost reservoirpressure. That is, to enable maximal boost reservoir charge benefits tobe incurred, the exhaust discharge valve may be closed before thereservoir is emptied and while the boost reservoir pressure is stillabove the exhaust pressure.

By the time the reservoir exhaust discharge valve is closed at t2, theexhaust pressure may be sufficiently high (e.g., higher than threshold610) and turbine spool-up may have been enabled. As a result, thecompressor boost pressure may also be sufficiently high. That is, turbolag may be reduced. As a result, after t2, the engine torque demand maybe met by the turbocharger compressor.

Now turning to FIG. 7, map 700 shows an example engine operation whereina boost reservoir is charged with high pressure EGR and then the highpressure EGR is delivered to the intake manifold during a transientrequest for increased EGR. Specifically, map 700 depicts a change inexhaust pressure at plot 702, a change in boost reservoir pressure atplot 704, an opening or closing state of a boost reservoir exhaustcharge valve (reservoir_exh) at plot 707, an opening or closing state ofa boost reservoir intake charge and discharge valve (reservoir_int) atplots 708-709, a change in EGR percentage in the boost reservoir at plot710, and a change in boost pressure at plot 712. In one example, theboost pressure may be estimated by a pressure sensor positioned in theintake manifold downstream of a turbocharger compressor, the exhaustpressure may be estimated by a pressure sensor in the exhaust manifoldupstream of the turbine, and the boost reservoir pressure may beestimated by a pressure sensor coupled to the boost reservoir. The EGRpercentage in the boost reservoir may be estimated by appropriatesensors or inferred based on engine operating conditions at a time ofreservoir charging and discharging.

Before t1, the engine may be operating with low boost pressure. Forexample, the engine may be operating un-boosted or at a low boost level.At t1, the boost level may be increased (plot 712), for example, inresponse to a tip-in, but may remain below a threshold level 713.Following t1, during a first engine cycle where the engine is boosted,but while the boost level is lower than threshold 713, the boostreservoir may be charged to a first pressure 705 with at least somecombusted exhaust gas from the exhaust manifold (plot 704).Specifically, a boost reservoir exhaust charge valve may be opened for aduration between t1 and t2 (plot 707). As a result of charging the boostreservoir with combusted exhaust gas from the exhaust manifold, an EGRpercentage of the reservoir charge may increase (plot 710).

At t2, during a second, later engine cycle, when the boost level ishigher than threshold 713 (plot 712), the boost reservoir may be furthercharged to a second higher pressure 706 with compressed intake air.Specifically, a boost reservoir intake charge valve may be opened for aduration between t2 and t3 (plot 708). Since boost pressure is used tofurther charge the reservoir, the boost pressure downstream of thecompressor may decrease (plot 712). As a result of charging the boostreservoir with compressed intake air from the intake manifold, an EGRpercentage of the reservoir charge may slightly decrease (plot 710).However, the slight decrease in EGR percentage is considered acceptablein view of the substantial gain in pressure. Thus, at t2, a highpressure exhaust gas and compressed air mixture may be generated andstored in the boost reservoir. As such, this may provide a source ofhigh pressure EGR that can be advantageously used to meet transient EGRdemands received while the engine is under boosted operation.

In this way, a controller may operate a turbocharger to provide anengine boost. Then, when the engine boost is lower than a threshold, thecontroller may charge the reservoir to a first pressure with at leastsome combusted exhaust gas from the exhaust manifold. Further, when theengine boost is higher than the threshold, the controller may charge thereservoir to a second, higher pressure with at least some compressedintake air from the intake manifold. As a result, a high pressure EGRmixture is stored in the boost reservoir. Specifically, even though theexhaust pressure is otherwise not high enough to charge the reservoir toa sufficient pressure for subsequent delivery to the engine intakeduring boosted engine operation, the addition of higher pressure intakegasses can raise the pressure, thus allowing at least some exhaust gasto be delivered to the intake, even when the engine is highly boosted.

At a later time (t4), such as during a third engine cycle following thesecond engine cycle, a transient request for increased EGR may bereceived. In response to this request, the pressurized charge at thesecond higher pressure 706 may be discharged from the boost reservoir tothe intake manifold. Specifically, a boost reservoir intake dischargevalve may be opened for a duration between t4 and t5 (plot 709, dashedline). The discharging during the third engine cycle may be performed inresponse to a tip-in event received during boosted engine operation, orin response to an EGR request received during boosted engine operation,for example. As such, during the third engine cycle, the boost level islower than the second pressure of the boost reservoir. That is, theboost pressure may not be higher than a pressure of the charge (herein,the high pressure EGR) pre-stored in the boost reservoir. By discharginghigh pressure EGR to the engine intake, high pressure EGR may be rapidlyprovided via a boost reservoir to improve combustion control and reduceNOx emissions. Specifically, EGR may be provided during boosted engineconditions when neither conventional high pressure EGR nor conventionallow pressure EGR can be rapidly and reliable delivered to the engine.

It will be appreciated that while the above example depicts pre-storinghigh pressure EGR and then providing the high pressure EGR in responseto a transient request for increased EGR, the engine controller may beconfigured to additionally provide low pressure EGR. Therein, exhaustgas may be recirculated via an EGR passage including an EGR valve, theEGR passage coupled between the intake manifold and the exhaust manifoldof the engine. Specifically, during a given engine cycle, the controllermay open the EGR valve of the EGR passage to recirculate exhaust gasfrom the exhaust manifold to the intake manifold (in comparison torecirculating higher pressure EGR via the boost reservoir over aplurality of engine cycles).

In this way, exhaust gas may be recirculated from an exhaust manifold,upstream of a turbine, to an intake manifold, downstream of acompressor, via a boost reservoir. Exhaust gas may be furtherrecirculated from the exhaust manifold, downstream of the turbine, tothe intake manifold, upstream of the compressor, via an EGR passage(that is, a low pressure EGR passage). Specifically, the exhaust gasrecirculated via the boost reservoir may be at a higher pressure (thatis, high pressure EGR) than the exhaust gas recirculated via the EGRpassage (that is, low pressure EGR). In this way, low pressure EGRbenefits and high pressure EGR benefits may both be achieved. Bypre-storing combusted exhaust in a boost reservoir, the pressurizedcharge can be discharged at a later time, when needed, to supplementconventional high pressure or low pressure EGR.

Now turning to FIG. 8, map 800 shows an example engine operation whereincylinder pressure is raised by discharging pressurized charge from aboost reservoir with the throttle closed while a compressor is run toraise boost pressure. Then, the throttle is opened and compressed air isdirected into the intake manifold. Specifically, map 800 depicts achange in throttle inlet pressure (TIP) at plot 802, a change inmanifold air pressure (MAP) at plot 804, a change in throttle positionat plot 806, an opening or closing state of a boost reservoir intakedischarge valve at plot 808, a change in pedal position at plot 809, achange in cylinder pressure at plot 810, and a change in boost reservoirpressure at plot 812. In one example, the throttle inlet pressure may beestimated by a pressure sensor positioned in the intake manifolddownstream of a turbocharger compressor and upstream of the air intakethrottle, the manifold pressure may be estimated by a pressure sensor inthe intake manifold downstream of the throttle, and the boost reservoirpressure may be estimated by a pressure sensor coupled to the boostreservoir.

Before t1, the engine may be operating with low TIP. For example, theengine may be operating un-boosted or at a low boost level. At t1, inresponse to a tip-in (as indicated by the change in pedal position atplot 809) a turbocharger compressor may be operated. As a result ofoperating the compressor, a boost pressure may start to slowly increase,as mirrored by the slow increase in TIP (plot 802). As such, for thecompressor pressure to be sufficiently high, rapid turbine spool-up isneeded. Until then, a turbo lag may be incurred. To enable higher boostpressures to be achieved more rapidly, at t1, during the tip-in,pressurized charge (including one or more of compressed air andcombusted exhaust gas) is discharged from the boost reservoir to theintake manifold, downstream of an intake throttle. The discharging isperformed for a duration (between t1 and t2) with the throttle heldclosed (plot 806). To discharge the pressurized charge from thereservoir, the intake discharge valve coupling the reservoir to theintake manifold is opened for the duration between t1 and t2 (plot 808).

As the boost reservoir is discharged (see drop in boost reservoirpressure at plot 812), a manifold pressure estimated downstream of thethrottle increases (plot 804). Further, a cylinder air charge increases(plot 810). This pressurized air allows an engine torque demand to bemet while the turbine spools up and while the compressor spins up toprovide the desired boost pressure. In addition, by holding the throttleclosed while the compressor is operating, boost pressure may bepre-charged. Specifically, a boost pressure (and thus TIP) may be raisedto a threshold value faster than would be otherwise possible. As can beseen at plot 802, between t1 and t2, the compressor pressure (mirroredby TIP while the throttle is closed) increases at a first, slower rateas the turbine slowly spools up to spin the compressor, and thenincreases at a second, faster rate as the turbine spins up faster and asboost pressure accumulates upstream of the closed throttle. As such,this operation with the throttle closed allows turbo lag to be reduced,as shown by a slower rate of attained the threshold TIP in the absenceof discharging from a reservoir with the throttle closed (plot 803,dashed line).

At t2, the pressure upstream of the throttle (TIP) may be at or above athreshold. Therefore, after the duration, at t2, discharging from thereservoir may be discontinued (plot 808) and compressed air may bedirected from the compressor to the intake manifold with the intakethrottle open (plot 806). Specifically, the intake throttle may beopened from the previous closed position and the compressor boost (thatwas being pre-charged upstream of the closed throttle) may be directedinto the intake manifold. As a result, MAP may rapidly increase andcylinder air charge can also rapidly increase. In this way, turbo lag isreduced while boost pressures are rapidly attained by temporarilydischarging pressurized air from a boost reservoir to an intake manifoldwith the throttle closed.

In this way, a boost reservoir may be advantageously used to storepressurized charge including compressed air and/or combusted exhaust gasfor subsequent delivery. Based on engine conditions, the boost reservoirmay be charged to achieve a desired charge pressure and EGR percentage.By pre-storing an amount of intake air and/or combusted exhaust gas in areservoir and discharging into the engine intake or exhaust manifoldbased on operating conditions, turbo lag may be reduced even if boost isalready present. By providing the charge to the intake manifold duringsome conditions, turbo lag can be reduced while meeting interim enginetorque demands. By providing the charge to the exhaust manifold duringother conditions, charge usage can be spread over a longer duration asthe turbine consumes the charge at a lower rate than the engineinduction. As a result, the increased exhaust pressure can assist incompensating for turbo-lag, and maintain a continuously increasingengine output while responding to a tip-in. By mixing combusted exhaustgas with compressed intake air in the reservoir, high pressure EGR maybe generated and stored in the reservoir for delivery to the intakemanifold, even during highly boosted engine operation. The dischargedhigh pressure EGR can improve combustion control and reduced NOxemissions during boosted operation. By pre-charging a compressor boostair with the throttle closed, TIP may be raised faster than would beotherwise possible. By allowing boost air from the reservoir to bedischarged into the intake while a pressure of the compressor boost airis raised, turbo lag may be better addressed while also meeting thetorque demands. Overall, boosted engine performance is improved.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for a turbocharged engine, comprising: during lower boostconditions, charging a boost reservoir with combusted exhaust gas to afirst pressure; during higher boost conditions, raising the boostreservoir to a second, higher pressure by further charging the boostreservoir with compressed intake gas; and discharging the pressurizedgas from the boost reservoir to an engine manifold.
 2. The method ofclaim 1, wherein charging the boost reservoir with combusted exhaust gasincludes charging with one or more of low pressure EGR, high pressureEGR, and combusted exhaust gas received directly from the exhaustmanifold via a valve.
 3. The method of claim 1, wherein the dischargingincludes discharging in response to a transient request for increasedEGR.
 4. The method of claim 3, wherein the transient request forincreased EGR includes a tip-in event while a boost level is higher thana threshold level.
 5. The method of claim 1, wherein discharging to anengine manifold includes discharging to an engine intake manifold. 6.The method of claim 1, wherein the discharging includes, discharging inresponse to a request for EGR during boosted engine operation when boostpressure is lower than the second pressure.
 7. The method of claim 6,wherein the discharging further includes discharging until the boostpressure is at the second pressure, and then discontinuing thedischarging.
 8. The method of claim 1, wherein charging with combustedexhaust gas includes selectively opening a first valve coupled betweenthe boost reservoir and an exhaust manifold, wherein further chargingwith compressed intake gas includes selectively opening a second valvecoupled between the boost reservoir and the intake manifold, and whereinthe discharging includes selectively opening a third valve coupledbetween the boost reservoir and the intake manifold.
 9. The method ofclaim 8, wherein the second valve is coupled between the boost reservoirand the intake manifold upstream of an intake throttle, and wherein thethird valve is coupled between the boost reservoir and the intakemanifold downstream of the intake throttle.
 10. The method of claim 1,wherein charging the boost reservoir with air and combusted exhaust gasincludes charging with a ratio of intake air to combusted exhaust gas toprovide a desired boost reservoir EGR percentage.
 11. The method ofclaim 1, wherein discharging to the intake manifold includes dischargingdownstream of a turbocharger compressor and downstream of an intakethrottle.
 12. The method of claim 1, further comprising, closing anintake throttle while discharging the pressurized gas from the boostreservoir to the intake manifold.
 13. A method for a turbochargedengine, comprising: recirculating combusted exhaust gas from an exhaustmanifold, upstream of a turbine, to an intake manifold, downstream of acompressor, via a boost reservoir.
 14. The method of claim 13, furthercomprising, further recirculating combusted exhaust gas from the exhaustmanifold, downstream of the turbine, to the intake manifold, upstream ofthe compressor, via an EGR passage.
 15. The method of claim 14, whereinthe exhaust gas recirculated via the boost reservoir is at a higherpressure than the exhaust gas recirculated via the EGR passage
 16. Themethod of claim 13, wherein recirculating via the boost reservoirincludes, during a first engine cycle, when a boost level is lower thana threshold, charging the boost reservoir to a first pressure with atleast some combusted exhaust gas from the exhaust manifold; during asecond, later engine cycle, when the boost level is higher than thethreshold, further charging the boost reservoir to a second higherpressure with compressed intake air; and during a third engine cyclefollowing the second engine cycle, discharging the pressurized chargefrom the boost reservoir to the intake manifold.
 17. The method of claim16, wherein during the third engine cycle, the boost level is lower thanthe second pressure of the boost reservoir.
 18. The method of claim 13,wherein the discharging during the third engine cycle is in response toa tip-in event, or in response to an EGR request received during boostedengine operation.
 19. The method of claim 13, wherein recirculating viathe EGR passage includes, during a given engine cycle, opening an EGRvalve of the EGR passage to recirculate exhaust gas from the exhaustmanifold to the intake manifold.
 20. An engine system, comprising: anengine including an intake manifold and an exhaust manifold; aturbocharger including a compressor and a turbine; a boost air reservoircoupled to, and configured to receive a charge from, each of the intakemanifold and the exhaust manifold; an EGR passage coupled between theexhaust manifold and the intake manifold, the EGR passage including anEGR valve; and a controller with computer readable instructions for,operating the turbocharger to provide an engine boost; when the engineboost is lower than a threshold, charging the reservoir to a firstpressure with at least some combusted exhaust gas from the exhaustmanifold; when the engine boost is higher than the threshold, chargingthe reservoir to a second, higher pressure with at least some compressedintake air from the intake manifold.
 21. The system of claim 20, whereinthe controller includes further instructions for, while the engine boostis higher than the threshold, in response to an EGR request, dischargingpressurized charge from the boost reservoir into the intake manifold,downstream of the compressor; and while the engine boost is lower thanthe threshold, in response to an EGR request, opening the EGR valve torecirculate combusted exhaust gas from the exhaust manifold to theintake manifold.